The Carver Challenge Page 2

• The reference amplifier should not have a higher power-output capability than the Carver. If it had, Bob would be obliged to beef up his own power supply, which would take additional time and prove nothing. (If cost-effectiveness is no consideration, any knowledgeable designer can put together a "perfect" power supply, given time and the necessary parts.)

• If we felt there was any audible difference between the amplifiers, Bob would be allowed 48 hours to eliminate that difference. If he proved unable to do so within that time, we would declare the game over and him the loser.

• If Bob felt that he had duplicated the reference amplifier, and we still heard differences, we would be subjected to a blind A/B test in which the only criterion would be whether we identified the reference amplifier correctly more than 50% of the time.

Because none of us figured that this project would be rapidly concluded, we had reserved a room for Bob in Santa Fe's La Posada hotel. After Bob and his 15 numbered cardboard cartons of equipment were settled in, we unboxed one of his M1.0 amplifiers and headed to my place for some preliminary listening.

We were pleasantly surprised. The Carver amp had none of the usual "mass-fi" solid-state hardness, but was, in fact, very listenable, with good depth, quite good detail, and only a modicum of that high-end dryness and laid-back midrange which characterize medium-priced solid-state amps.

Not surprisingly, the reference amplifier sounded very different and, in our opinion (shared, in most respects, by Bob), much better. We noted, with interest, that he immediately heard every difference that we observed between his amp and the reference.

The Approach
I had assumed that Bob would simply listen at length to our reference amplifier, make a measurement or two, then try various means to duplicate what he had heard and measured. His approach turned out to be much less scattershot than that. I don't think we had listened for more than an hour when Bob suggested that he "get to work." We transported him and the two amplifiers back to his room, leaving him to his own devices for the rest of the day.

Next morning, Bob called to say he had something for us to hear. How soon? As soon as we could get down to his room.

The hotel room was a shambles! Across one end was a long table buried in oscilloscopes, distortion analyzers, voltmeters, the two amplifiers, a soldering iron, a white noise generator, two unidentifiable chasses full of inductors, resistors, and capacitors, a large table fan (there was no air conditioning), a half-dozen partially-drained Diet Coke cans, and perhaps 50 feet of audio cables, test leads, and clip-lead interconnects. The adjacent sofa and table were covered with countless little plastic bags of resistors and capacitors, several schematic diagrams, and sheets of paper crammed with arcane numbers and calculations. On the floor under the table was a Rogers LS3/5a loudspeaker which appeared to be connected to both amplifiers at once.

Bob explained that this would be a different kind of listening test. We would not be listening to his modified 1.0 or our own reference amplifier. We would be listening to the difference between them. He ex- plained that he had tacked two identical loads, each approximating a loudspeaker, to one channel each of his and our amplifiers. He had then connected the LS 3/5A and a sensitive voltmeter between the Hot or Plus terminals going into those dummy loads. This simple hookup would allow him to hear and measure the amplitude of any differences between the signals appearing at the amplifier outputs.

If both amplifiers had exactly the same gain (amplification), and were fed exactly the same signals to their outputs at exactly the same instant, the signals appearing at one amp's Plus terminal would be exactly the same as those appearing at the other amp's Plus terminal. That is, there would be no voltage difference between those terminals, and no signal would appear across the monitoring loudspeaker and voltmeter. No sound would be heard and no voltage would be read on the meter. Any sound, or voltage reading, would thus reflect a difference between the signals at the amplifier outputs—a difference which is was now Bob's stated objective to eliminate.

In essence, this is a test of the ability of one amplifier (the Carver) to cancel the output signal of the other (the Reference). Or, as Bob expressed it, to compare the transfer functions of the two.

A transfer function is nothing more than a statement of the relationship between the signal fed into a device and the signal that comes out of it. For example, a frequency-response specification is a description of the transfer function telling us how much an input signal of fixed amplitude and varying frequency will vary in amplitude at the output.

Bob's test hookup would show much more than frequency response differences. In fact, one of his most interesting statements, for those of the "every amplifier is the same except for frequency response" school, was that varying frequency response between the 1.0 and the reference amp made up only about 25% of the significant differences. Relative phase shift, source impedances (damping factors)—in short, every electrical difference between the amplifiers—would produce a signal at that test point between the Plus output terminals. When the amplifier outputs were identical, in all respects, there would be total cancellation—a null—of the difference signal. Bob's goal was a 70dB null, or an 0.03% difference between the two amps.

Just to indicate how ambitions a goal this is, Bob quoted a figure of 48dB as the null you might normally hope to product between two channels of the same amplifier! The meter would measure the voltage difference between the two hot terminals, and thereby the degree of cancellation in decibels; the speaker would reproduce this signal to give an idea of how audibly significant the differences were. (To check how loudly the music produced the difference signal, you had but to disconnect one of the hot leads; in that situation there was a 0dB null.)

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